A Southern California man has been arrested on suspicion of having an unregistered destructive device amid the investigation into the explosion that killed his ex-girlfriend at the day spa she owned, according to the FBI and a criminal complaint.

But Stephen Beal, 59, has not been charged in connection with the Tuesday explosion that killed his ex, 48-year-old Ildiko Krajnyak, who was also his business partner, and injured three people, federal authorities cautioned.

Inside the Aliso Viejo, California, crime scene, agents said they found items consistent with an explosive device: a 9-volt battery, a cellphone, loose wires melted material that appeared to be duct tape, according to the criminal complaint against Beal.

Injured victims saw Krajnyak pick up a box in a pile of mail and open it, after which it exploded, according to the complaint.

Zubin Ghafari-Saravi

The aftermath of an explosion at a building in Aliso Viejo, California, May 15, 2018.

Raul Hernandez

The scene of an explosion in Aliso Viejo, Calif., is pictured May 15, 2018.

During the investigation into the explosion, officials served a search warrant at Beal's Long Beach, California, home. Multiple explosive materials were found in Beal's home, including at least three containers of black powder, a 7-foot-tall rocket, rocket-making equipment, two containers of potassium perchlorate -- used as a precursor -- and two containers of red gum, which is used as fuel and binder in fireworks, according to the complaint.

Altogether, 130 pounds of explosives/precursor material were found in his home, more of which was discovered in the garage, the complaint states. Beal admitted to owning the items, claiming to be a model rocket hobbyist. But, he has allegedly not pursued that hobby since 2004, according to the court document.

Beal appeared to have rented space in the building with Krajnyak, according to the criminal complaint. The pair met in 2016 through an online dating service, according to the complaint. They began dating and then opened the spa together, the complaint states.

While Beal and Krajnyak only dated for a year and a half, they remained business partners, according to the complaint. Beal paid the rent -- $1,500 per month -- and half of the spa's operating costs, the complaint states. Some months, Beal would have to loan Krajnyak money to cover all of the costs, and sometimes she would make enough to cover the costs, according to the complaint.

The spa was originally located in a different building and relocated to its current building a few months ago, the complaint states.

Their relationship allegedly became strained due to financial issues and disputes over the exclusivity of the relationship, the complaint states.

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Orange County first responders at the scene of an explosion in Aliso Viejo, Calif., May 15, 2018.

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Firefighters and first responders at the scene of a building explosion in Aliso Viejo, Calif., May 15, 2018.

Beal was set to make his first appearance in U.S. District Court in Santa Ana this afternoon.

The crime scene at the day spa has been rendered safe after the blast sent plumes of white smoke into the air and debris flying into the parking lot.

While evidence collected from the blast suggests that the incident was intentional, the exact means by which it was carried out has not yet been determined, according to the complaint.

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The scene of an explosion at an medical building in Aliso Viejo, Calif., is pictured May 15, 2018.

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Firefighters and first responders at the scene of an explosion in Aliso Viejo, Calif., May 15, 2018.

The family and friends of Krajnyak are in "complete shock," Irene White, a friend of the family, told reporters Wednesday on the family's behalf.

"Families always came first for Ildiko," White said. "She took great pride in finding beauty in everyone. At this time our family and friends would like to privately mourn a mother, a wife, a daughter and a friend."

White said the family is hoping to get answers.

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People are evacuated after a building explosion in Aliso Viejo, Calif., May 15, 2018.

Mindy Schauer/The Orange County Register/SCNG via AP

Firefighters and sheriff's deputies escort children from Academy on the Hill pre-k school in Aliso Viejo, Calif., May 15, 2018, after a fatal explosion nearby.